Allow Users to have custom themes

[JakobDev]

Feature Description

At the moment, it's only possible to add themes for a Gitea installation. If you are just a User and not the Admin, you can't add a Theme. The only Way to use a custom theme is asking the Admin, if he could add it or by using a Extension like Stylus. I think we should add the possibility for Users to set a custom theme. Maybe by adding a link to a CSS in the Settings.

Screenshots

No response

[silverwind]

Would just offer a textarea for a user to paste CSS which is a lower barrier of entry than requiring external hosting and also has no issues with CSP. The user can still opt to just @import a external stylesheet if they so desire.

That said, theming is still not quite where I like it to be (CSS vars only), we still need to variablize the chroma and codemirror colors.

[Mosch0512]

This would be great.